What Is Piping – Page 158 – A Blog to learn Piping, Mechanical, and Process Engineering Basics in a Simple Way

Types of Mechanical Draft Cooling Towers The forced draft is a mechanical draft (Fig. 1) towerblower type fan at the intakeFan forces air into the tower, creating high entering and low exiting air velocities.Induced draft, a mechanical draft towerwith a fan at the discharge which pulls air through the towerfan induces hot moist air out the dischargelow entering and high exiting air velocities, reducing the possibility of recirculation in which discharged air flows back into the air intakeTwo configurations, Cross Flow and Counter Flow Fig. 1: Mechanical Draft Towers Cross Flow Induced Draft Cooling Towers Airflow directed perpendicular to the water flowAirflow enters one or more vertical faces of the cooling tower to meet the fill materialAir continues through the fill and thus past the water flow into an open plenum areaWater flows (perpendicular to the air) through the fill by gravityGravity distributes the water through the nozzles uniformly across the fill material Counter Flow Induced Draft Cooling Towers The airflow is directly opposite of the water flowAirflow first enters an open area beneath the fill media and is then drawn up verticallyThe water is sprayed through pressurized nozzles and flows downward through the fill, opposite to the airflow Advantages of Cross Flow Towers Low pumping head, lower first cost pumping systemsLower energy and operating costsAccepts larger variation in water flow without adverse effect on the water distribution systemEasy maintenance access to distribution nozzlesLow static pressure dropReduced drift, less make-upMore air per fan horsepowerLarge diameter fans can be used so that fewer cells are required for a given capacityDue to more fill gap choking is minimizedCost lower than counterflow Disadvantages of Cross Flow Towers Larger footprintApproach to cooling tower limited to 4 deg CThe icing of louvers during cold weather, Larger louver surface area makes icing more difficult to controlLow-pressure head on the distribution pan may encourage orifice clogging and less water breakup at the spray nozzle Advantages of Counter Flow Towers Increased tower height accommodates longer ranges and closer approaches (can be less than 4 deg C)More efficient use of air due to finer droplet size from pressure spraysVertical air movement across the fill allows the coldest water to be in contact with the driest air maximizing tower performance Disadvantages of Counter Flow Towers Cost higher than Crossflow towerHeight is moreIncreased system pumping head requirementsIncreased energy consumption and operating costsDifficult to inspect and clean distribution nozzlesRequires individual risers for each cell, increasing external piping costsFill gap is less so choking is possibleResistance to upward air travel against the falling water results in higher static pressure loss and a greater fan horsepower than in crossflow towersRestricted louver area at the base with the high velocity of inlet air increases the fan horsepowerThe tendency for uneven distribution of air through the fill with very little movement near the walls and center of the towerHigh inlet velocities are liable to suck airborne trash and dirt into the tower When to choose – Crossflow towers? To minimize pump headTo minimize pumping and piping first costsTo minimize operating costsWhen flow variance is expected from the processWhen ease of maintenance is a concern When to choose – Counterflow towers? When space (footprint) is a concernWhen icing is of extreme concernWhen pumping is designed for additional pressure drop Material Balance of Cooling Towers A water balance around the entire system is: M = E + B + W (where, E = Evaporation, B = Blowdown and W = Windage losses, in m3/hr)Evaporation Losses, E = {C X (T1- T2) X Cp} / Hv (where, C = amount of circulating water in m3/hr, T1/T2 = return/supply temperature, Cp = specific heat of water and Hv = latent heat of vaporization of water)Windage or drift losses, W from large-scale industrial cooling towers, in absence of manufacturer’s data, may be assumed to be:W = 0.3 to 1.0 percent of C for a natural draft cooling tower without windage drift eliminatorsW = 0.1 to 0.3 percent of C for an induced draft cooling tower without windage drift eliminatorsW = about 0.005 percent of C (or less) if the cooling tower has windage drift eliminatorsSince the evaporated water has no salts, a chloride balance around the system is: M (Xm) = B (Xc) + W (Xc) = Xc (B + W) where, M = Make-up quantity in m3/hr, Xm = concentration of chlorides in make-up water in ppmw and Xc = concentration of chlorides in circulating water in ppmw.Xc/Xm = Cycles of concentration = ppm chloride in circulating water/ppm chloride in make-up water = M / (B+ W) = M / (M – E) = 1 + { E / (B + W) }Lower COC means high operating cost due to higher make-up As the cycles of concentration increase the water may not be able to hold the minerals in solution, once solubility is exceeded, they precipitate and cause fouling in exchangersCycles of concentration vary from 3 to 5 depending upon the make-up water quality Components of a Cooling Tower Cooling Tower Hot Water distribution system – The collection header and branch headers to the individual cells via flow control valvesCooling Tower Fans – with on-off, dual speed, variable speed motorsCooling Tower Fill, Louvers and Drift Eliminators – Each fill sheet has louvers and drift eliminatorsCooling Tower Cold Water Basin – Screens, Sluice gates, pump suction linesTypes of pump used – Horizontal Centrifugal, Vertical turbineFill, the heat transfer surface, most important component of a cooling towerThe efficiency of the tower depends upon its ability to promote both the maximum contact surface and the maximum contact time between air and waterTwo basic types of fill – Splash type and Film typeIn crossflow, either type can be applied but in counterflow, through either type can be used but tends toward almost exclusive use of the film fills Material of Construction of Cooling Towers Wood: Available, Workable, Low Cost, and Durable.Metals: Steel, Cast Iron, copper alloys, and aluminum alloys. Special care required for corrosion control for metals other than steel. Used for fan hubs, basins, partitions, fan cylinders, bolts, nuts, washers, gear cases, anchor castings, etc.Plastics: Capable to be molded into single parts of complex shape and dimensions. Used for fan blades, fan cylinders, fill supports, drift eliminators, louvers, etc.Concrete: Higher initial cost but decreased fire risk and higher load-carrying capacity. Special Types of Cooling Towers Dry Cooling TowersHybrid Cooling Towers (also called plume abatement towers) Dry Cooling Towers Basically an air-cooled heat exchangerSensible cooling of water through finned coilsThe low temperature of inlet air is essentialLess efficient cooling – large surface areasWater temperatures higher than evaporative type cooling towers Hybrid Cooling Towers Combines both dry and wet sectionsOperational flexibility in case of variation in ambient conditionsMore effective cooling than dry type towersAvoids plume formation Hybrid Cooling Towers-Sub Types Parallel path towerSeries path towerAdiabatic air pre-cooler Parallel Path Towers (Fig. 2): Coil section located vertically on top of the fill sectionInduced draft fanHot water flows through the coil and fills section in seriesParallel streams of airflow through the coil and fill sectionsDry and saturated air streams mix in the fan section Fig. 2: Parallel Path Towers The ratio of two sections depending on the inlet air characteristicsSupply of air to each section may be adjusted with louvers at the inletDuring low ambient temperatures fill section may be completely isolatedRequires more height of the tower Series Path Tower: Air flows through the coil and fills sections in seriesInduced draft fan – cross flowThe coil may be before or after fill sectionDe-saturation of the air stream in the coil sectionLess height of tower compared to parallel path towersLess flexibility of operation on airsideThe proximity of the dry coil to fill section leads to:ImpingementScaling problemsRestricted airflow. Adiabatic Air Pre-Cooler (Fig. 3): Also known as a humidified air coolerEffective for low RH areasFill section adjacent to dry section with drift eliminator in betweenFinned tube bundle located on top of dry sectionThe induced draft fan draws air through fill and coil sections in seriesTwo water streams – closed loop through the coil and open loop through the fill Fig. 3: Adiabatic Air Pre-Cooler Water re-circulated infill sectionAir, cooled in the fill section, exits in the saturated conditionProcess water-cooled in the coil with outlet air from fill sectionAir at the outlet of coil section exits at a higher temperature and in unsaturated conditionWater circuit in an open loop can be shut-off during winterLess impingementExisting evaporative towers can be retrofitted Disadvantages: Two water circuits mean additional accessories and chemical treatmentOverall space requirement increases Comparison of Hybrid and Evaporative Towers Hybrid towers have: Fewer problems of water lossNo plume formationMore flexibility in operationBigger sizesMore accessoriesHigher fan powers